Author Topic: QB PCP (long)  (Read 7239 times)

Offline ribbonstone

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QB PCP (long)
« on: March 18, 2010, 02:41:45 PM »
They really tried for power too hard.

Out of the box, the  .177 QB PCP looks good; finish and stocking is much better than the QBs of just a couple of yeas ago.  The new 3K air tube isnt quite as nicely finished  as the China made parts, the  polish isn't as good and the blue seems thinner, but only time will tell how well it wears.


This is the gun just as it came out of the box,,,no buffing or oiling, exactly as it was shipped.

If you insist on a Discovery comparison, then over all the polish and blue of the QB is a little ahead of the Discovery but not enough to get excited about, and if the air tube blue on the QB wears quickly, then the Discovery would be ahead.  Stock of the QB is much better shaped for a full sized adult, has a nicer line and feel, and although of some Asian crap wood, this example has a nice grain under the toe-nail polish type finish.

Basically, as many of the QB78 co2 parts as possible were used.  The adjustable QB78 trigger, receiver, bolt, barrel, stock, etc.  Everything except the air tube, fill assembly, some screws, and some valve parts are pure QB.

Stock is cut for the barrel band and the barrel slotted, but the barrel band is used near the muzzle and not in the traditional spot, so these cuts do not serve a function on the PCP version (although I was glad to see them as I did use them with a 2nd band).  

There are two added valve retention screws.  This is nice, but remember that the QB system has a large steel ring inside the tube the valve butts up against.  This ring takes the large stock bolt from below and the breech screw from above, so its pretty well retained even without the addition of the two added screws.

Not shown, the stock valve retention screw is replaced, and oddly it takes a different sized allen key than the two visible valve retention screws.

Going to have to guess on air volume  The tube is a little shorter than a Discovery, so the volume is probably close to 125CCs.

First test:
I did run a few patches through the bore, but no other prep.  Filled to 205BAR (almost 3K).

Bolt is very hard to close due to major spring tension but was smooth enough to show someone had gotten most of the worst burrs out of the system.

.177 10.6gr. Kodiak


That spring tension to the striker is set way-way too high.  When the vel. drops from the first shot at max. pressure, the spring tension is too much.  If this gun could be filled to 3.5K, it maight make for a decent shot count, but as it is, THE SHOT COUNT SUXS.

But it does get decently close to the advertized velocity.  If it can get 950fps with 10.6gr. pellets (for  21.2 foot pounds) it will get about 1150fps with light weight LEAD pellets.


1.   The bolt is hard to close because the spring tension to the striker is way-way high.  Higher than can be used with a 3K fill.
2.   Its loud.
3.   The barrel is NOT uniformly smooth. Are a could of loose spots inside, but luckly none of them are near the muzzle.  It should shoot well.
4.   There is a burr on the muzzle.

Kiss the warranty good-by.  Taking the rifle apart and changing a few things.

If you've been inside a QB co2 gun, there aren’t a lot of surprises.  I did NOT take the valve or the fill assembly out of the tube; those parts are still a mystery as of now.

The tube is slightly larger than standard and the metal barrel band used had to be lapped to fit the new tube.  It is a very tight fit and needed to be driven off with a brass drift (this is good as a sloppy band will not do its job worth a crap).

Standard striker, bolt, etc.  Will have to take the bolt out later to see if there was any mods made to the hollow bolt probe or not; LOOKS stock on the outside, but suspect it was reamed on the inside to a slightly larger air passage.

The Hard cocking is because of a major spring change.  Striker parts nicely smooth and polished, which really helps the smoothness.

Spring  just looks longer.
But it is a TWO PIECE spring. Why theyd do this and not just use a new spring is beyond me.  It does use the factory spring, but cutting a new short spring to fit behind it has to be labor intensive.  How that is cheaper than just getting a new longer spring, and not having to cut and fit a "spring chunk" is a mystery.

Tossed the add on "spring chunk". Need a lighter striker hit to bring the sweet spot down, so less spring would be a good thing for lowering the pressure level of the sweet spot.

Used a .22 brass spacer in place of the spring chunk.

NOTICE: with this spacer, 8 coils stick out past the guide.   With the original 2-piece spring, 12 coils stuck out past the guide.

Tame the bark:
 Had a front sight assembly from an AR2078.  If you reverse it, so the long part sticks out front, can use that long hollow section for adding various muzzle adaptors.  The sight is split, and is clamped by the cross screw.

Rather than use the issue pieces (which would require a slot cut in the barrel for the large pin ), just used a 6X32 screw and nut (will get a blued one later).  Can clamp it so it won’t move, but remove it without cutting on the barrel.
Made three attachments, a 2" long stump, a 4"  long  version, and a 6" long version. For what I was doing, the 4 " long version worked fine (but remember, these fit onto the hollow section of the sight assembly, so there is more volume than length alone would account for.

Had a spare barrel band, so added it to the  existing cuts in the barrel/stock (right where it would have been on a CO2 QB).
With a spare 3-9X scope, was ready for a 2nd (lower power) Test.


Same 10.6gr. .177 Kodiaks

STILL starting its sweet spot at nearly 3K (if I had filled to 3K rather than 2.9K, suspect there would have been another 4 or 6 sweet spot shots.  Would expect about 30 shots in a 25PS variation string if starting at 3K.

Will detune the spring a little more, hoping to get the sweet spot to start at 2900-2800psi. may lose a little power, but expect to get the shot count closer to 30shots.

That 17 foot pounds doesnt sound like a whole lot, but that would be coming close to 1000fps with standard (7.7-8gr.) pellets.
3rd test (light failing for the day):

Did a fast-n-dirty re-crown using hand tools, mostly just removing one little offending burr.

Pumped the rifle back to 2600PSI and fired 10 shots at 20yards (still using 10.6gr. Kodiaks)

Thats right at 4/10ths for 10 shots.
Will be other posts, as Ive got to lathe recrown that barrel right, get the valve out to see how they modded it,  adjust the trigger a bit better, etc.

All in all, am pleased with the little rifle.  

But if you are not use to working on QBs, advice would be to buy a Discovery.

Offline spysir

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nice write up,
« Reply #1 on: March 19, 2010, 03:58:06 PM »
Thank you for sharing. From you pictures (I didnt click for larger as I am on dial-up) I cant tell if the extra valve anchor screws are counter sunk or if the threads are holding it? Either way should be okay as you are right it is anchored pretty well either way. On my 12fpe HPA QB a couple of posts down extra anchors were used, seemed a good idea while there I reckon. To me it really seems whom ever put this thing together for Compasco put very little (if any) R&D into it which is a shame as I own both the Disco and a QB78/79 and the QB trigger is the big advantage between the two. Perhaps they tought everyone knows how and likes to tinker with QB's but with just a bit of work such as you have done the new QB pcp would be one fine rig. Maybe they will read you posts and fix things up?

 Please keep us updated.


Offline ribbonstone

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« Reply #2 on: March 19, 2010, 04:17:15 PM »
Also an HPA QB user.

The PCP QB has decent sized retention screws and look to be of a good grade, but aren’t countersunk.  Considering that there are 3 retention screws and that internal steel ring with it’s two screws, believe the valve is staying put.

Used retention screws on one of the QB HPA conversions (the .25 below…one on he far left).  Didn’t use them on the other 3 HPA’s, the pressure used on them (850psi) isn’t any greater than co2 pressure on a mild day.


Offline jake

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RE: QB PCP (long)
« Reply #3 on: March 23, 2010, 05:04:33 AM »
nice write up! im sure thats going to help multiple people out!
if at first you dont succeed, re-load, kill count 2010.

Offline Jaymo

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Re: QB PCP (long)
« Reply #4 on: March 28, 2010, 03:31:01 PM »
While you have it apart, I say turn up some plugs to fit in the ends of the tube. Center drill them. Polish it between centers in the lathe and then hot blue it or blue it with Brownell's Oxpho Blue.
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Offline Gene_SC

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Re: QB PCP (long)
« Reply #5 on: March 28, 2010, 04:27:49 PM »
Long but very comprehensive Rib. You answered all my questions in your review. Thanks
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